1. Technical Field of the Invention
The present invention relates to a light beam scanning apparatus and an image forming apparatus, and in particular, to a light beam scanning apparatus in which a plurality of light beams, such as laser beams, are used to scan a single image carrying member for exposure thereof in forming an electrostatic latent image on the image carrying member and an image forming apparatus with this laser light scanning apparatus therein.
2. Related Art
Recently, a various types of imaging forming apparatuses, such as digital copying machines and laser printers, have been developed and are now already in practical use, in which exposure scanning using a laser light beam (hereinafter, simply called “light beams”) is combined with an electrophotography process to form images.
In the field of image forming apparatuses, there has recently been a stronger demand for forming images at higher speeds. As one means for responding to this demand, a multi-beam type of digital copying machine has been developed. This multi-beam technique enables a plurality of light beams to be generated to perform simultaneous scans along plural lines. Specifically, a plurality of semiconductor lasers are used to output plural light beams each guided to a photosensitive drum through reflections at several optical systems, and are scanned along the drum. In the image forming apparatuses each employing the multi-beam technique, pulse width modulation circuits, which produce drive signals by performing pulse width modulation (PWM) depending on image data, are disposed in driving paths for the plurality of semiconductor lasers, respectively.
On the other hand, a single-beam technique, which uses only one light beam for scanning, but is still able to cope with faster image formation, has been known as well. Examples based on this technique include the configuration disclosed by Japanese Patent Application No. 2004-168425. In this configuration, a single laser oscillator generating a scanning light beam is disposed and a plurality of transfer channels to the laser oscillator are formed to transfer image data thereto. Practically, a data processor is provided, in which image data supplied from a scanner section are subjected to predetermined image processing and digital image data of each line are distributed and outputted into two strings of image data (two data channels): a string of image data at odd-number-th pixels (called “odd pixels”) and a string of image data at even-number-th pixels (called “even pixels”). This data processor further includes two serially connected circuits each having a pulse width modulation circuit and a driver, which process the image data composing each string. Both the laser drivers, each belonging to each data channel, have output terminals electrically connected to the single laser oscillator via, for example, a wired logical add (OR) circuit. In this circuitry, timings at which both the pulse width modulation circuits for the odd and even pixels output modulated signals respectively are mutually shifted by half an operation cycle of this circuit. This allows the pulse width modulation circuits to output the modulated signals at a speed which is double as fast as a maximum rated operation speed of each pulse width modulation circuit, thus forming images faster.
However, in any of the foregoing multi-beam and single-beam techniques, two or more pulse width modulation circuits are absolutely necessary. In particular, in the case of the multi-beam technique, two or more sets of circuits each including a pulse width modulation circuit, driver and laser are absolutely necessary. This means that there is larger influence of individual differences of the components. That is, even if instructions of the same amount are given, there are hardly provided PWM-modulated drive signals of the same pulse width, due to various factors such as individual differences of the pulse width modulation circuits, differences of positions at which light beams pass lenses, and errors in transmittance. That is, even though the same command values are given, accuracy in scanning is forced to be lowered and deterioration in the quality of images to be formed is inevitable as well. The above-said drawbacks due to the individual differences invite an increase in the rate of an error per dot, as the speed of the image forming operation becomes faster.